When he assumed the gown of manhood, he dreamt that Fortune said she was tired of standing before his door, and that unless she were quickly admitted, she would fall a prey to the first comer. When he awoke, opening the door of the hall, he found close by the threshold a bronze statue of Fortune more than a cubit high. This he carried in his arms to Tusculum, where he usually spent the summer, and consecrated it in a room of his house; and from that time on he honoured it with monthly sacrifices and a yearly vigil.

For it seemed to him in a vision that Fortune told him that she had now remained by him for a long time, yet no one would grant her admission into his house, and that, if she should be barred out much longer, she would take up her abode with somebody else.

The parallel from Aeschylus doesn't seem too close, but the others are interesting, especially those about Galba. I don't have any commentaries on Revelation handy. If I'm not mistaken, the ancients knocked on doors with a kick of the foot, not a rap of the knuckles.

Update from Kevin via email, who cites Song of Songs 5:2 with a comment:

I was asleep but my heart was awake.A voice! My beloved was knocking:"Open to me, my sister, my darling,My dove, my perfect one!For my head is drenched with dew,My locks with the damp of the night."

[New American Standard Bible translation]

Both Jewish and Christian traditions have understood the knocking lover as God. Whether that was the original intention of the author could be a knock-down -- or knock-up -- argument.